1. Field of the Invention
The invention relates to a data transmission system wherein the binary data is transmitted in the form of multi-level differential signals over respective transmission channels, and the received signals are decoded on the basis of the signs of the differences between pairs of such signals rather than their magnitudes. A high transmission rate and immunity from common mode noise is thereby achieved, and with fewer channels than would be required for differential transmission in binary form. As used herein the term "multi-level" refers to more than two possible levels, two levels being characteristic of binary signals.
2. Description of the Related Art
Transmission of binary data is most commonly effected by employing a binary single-ended interface, the signals in the respective transmission channels corresponding to respective bits of the binary data. The group of signals on all channels is commonly referred to as an "interface", and in the absence of noise a binary single-ended interface makes the most efficient use of channel capacity. Such interfaces are, however, subject to transmission error when there is common mode noise on the channels; i.e., noise occurring substantially to the same extent on all channels, since decoding of the received signals requires detection of their amplitudes relative to a fixed reference level.
Multi-level rather than binary interfaces have been employed in order to obtain an increased information transmission rate over a given number of channels, the value of the signal in each channel being at any of several possible levels which respectively correspond to several binary data bits. See, for example, U.S. Pat. No. 4,606,046, issued Apr. 12, 1986. However, such interfaces still encounter the common mode noise problem, since decoding of the received multi-levels requires comparison with a fixed reference level.
It has also been recognized that the problem of common mode noise in transmission can be substantially eliminated by encoding the binary data in differential form, each bit thereof being represented by the difference between signals on a pair of lines, since such a difference will not be affected by equal changes in the amplitudes of the signals on both lines. Also, only the sign (rather than the magnitude) of the signal difference need be detected. However, binary differential transmission necessitates double the number of lines for transmitting a given number of data bits, as well as double the number of pin connections at the encoder and decoder. That is a significant disadvantage when such circuits are included in an IC package comprising further circuits and there are only a limited number of available pin connections.
Consequently, there is need for a data transmission system which provides a high information transmission rate together with common mode noise suppression, and which requires fewer transmission channels than necessary for binary differential transmission.